Active matrix substrate and organic EL display device

ABSTRACT

The present invention provides an active matrix substrate driven by an analog gray scale method, and an organic EL display device, in which decrease in response speed of a current emissive element is suppressed. The active matrix substrate of the present invention is driven by an analog gray scale method and is provided with a pixel that has a current emissive element and a transistor that supplies current to the current emissive element. The pixel further has a compensation circuit for compensating variability of threshold voltage in the transistor; the current emissive element has a pixel electrode electrically connected to the transistor; a gate electrode of a transistor that makes up the compensation circuit forms a region covered with the pixel electrode; and a part or the entirety of the gate electrode that is positioned within the region is provided in a wiring layer that is lower than a wiring layer directly below the pixel electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a U.S. National Phase patent application of PCT/JP2010/058980,filed May 27, 2010, which claims priority to Japanese Patent ApplicationNo. 2009-24139, filed Oct. 20, 2009, each of which is herebyincorporated by reference in the present disclosure in its entirety.

TECHNICAL FIELD

The present invention relates to an active matrix substrate and to anorganic EL display device. More specifically, the present inventionrelates to an active matrix substrate that is suitable for a displaydevice provided with a current emissive element such as an organic ELelement, and to an organic EL display device provided with the activematrix substrate.

BACKGROUND ART

Driving schemes in organic EL display devices include two types, passivematrix and active matrix schemes. Active matrix is the mainstreamdriving scheme at present, in particular in large display devices.

Ordinarily, the pixels in an active-matrix organic EL display devicecomprise each one organic EL element that is provided with a switchingtransistor for transmission of data signal, and with a drivingtransistor that drives the organic EL element on the basis of a datasignal that is transmitted by the switching transistor (for instance,Patent Document 1). Parasitic capacitance arises between these members,which are provided in the pixel, and wiring layers, for instancescanning lines, signal lines and the like. Methods have been proposed inwhich the display defect known as crosstalk, which derives from thisparasitic capacitance, is suppressed by arranging a metallic patternthat constitutes an electric field shield for scanning lines and signallines (for instance, Patent Document 2).

If the threshold voltage of the driving transistor of each pixelexhibits variability, driving of the driving transistor of each pixelwith the same gate voltage results in variability of the current valuethat is supplied from the driving transistor to the organic EL element.This is one cause of display unevenness. Known methods for solving thisproblem include, for instance, area gray scale expression ortime-division gray scale expression by digital gray scale driving. Inthe case of analog gray scale driving, methods have been disclosedwherein fluctuation in the threshold voltage of the driving transistoris detected for each pixel, and a so-called compensation circuit isformed that compensates that fluctuation (for instance, Patent Document3).

PRIOR ART REFERENCES

[Patent Documents]

Patent Document 1: JP 2006-47999 (page 2)

Patent Document 2: JP 2006-30635 (pages 2 to 4)

Patent Document 3: JP 2005-31630 (page 2 and 10)

DISCLOSURE OF THE INVENTION

The purpose of the abovementioned method of arranging a metallic patternthat constitutes an electric field shield was not lowering parasiticcapacitance inside pixels.

The above-described method of forming a compensation circuit is a pixelcircuit configuration and driving method that allows compensating athreshold value by itself, but the circuit configuration was complex,and parasitic capacitance formed inside the pixels, which gave rise topotential fluctuations that in turn detracted from response speed in thepanel.

The operation mechanism of a conventional organic EL display deviceprovided with a compensation circuit is explained next.

FIGS. 8 and 9 are plan-view schematic diagrams illustrating a pixel of aconventional organic EL display device provided with a compensationcircuit. The pixel has six transistors (T1 to T6), two capacitors (C1and C2), and one organic EL element OLED. In FIG. 8, scan[n−1] andscan[n] denote, respectively, an [n−1]-th and an [n]-th scanning line;Vini[n] denotes an [n]-th initialization voltage line; and em[n] denotesan [n]-th emission control line. The transistor T1 responds to a scansignal inputted from the scanning line scan[n−1], and a data signalstored in the capacitors C1, C2 is discharged via the initializationvoltage line Vini[n], to initialize as a result the gate voltage of thetransistor T4. The transistor T2 compensates variability in thethreshold voltage of the transistor T4. The transistor T3 responds tothe scan signal inputted from the scanning line scan[n], and performsswitching of the data signal inputted from the signal line data. Thetransistor T4 responds to the data signal inputted via the transistorT3, and decides the amount of current that is supplied to the organic ELelement OLED. The transistor T5 responds to the emission signal inputtedfrom the emission control line em[n], and performs switching of thecurrent that is supplied to the transistor T4 from the power source lineELVDD. The transistor T6 responds to the emission signal inputted fromthe emission control line em[n], and performs switching of the currentthat is supplied to the organic EL element OLED from the transistor T4.The capacitor C1 stores gate voltage that is inputted to the transistorT4. The purpose of the capacitor C2 is to assist the capacitor C1. Theorganic EL element OLED emits light in accordance with the current thatis supplied from the transistor T4. The anode of the organic EL elementOLED is connected to the drain of the transistor T6, and the cathode ofthe organic EL element OLED is connected to a power source line ELVSS.

Upon observation of response between gray scales in the organic ELdisplay device illustrated in FIGS. 8 and 9, a phenomenon of a step-likeresponse is observed wherein the original brightness cannot be reachedin a frame (one frame corresponds to a 16.7 ms display period)immediately after switching of gray scale, but is eventually reached insubsequent frames.

FIG. 10 is a graph illustrating measurement results of responsecharacteristic of a conventional organic EL display device provided witha compensation circuit. FIG. 10 illustrates results of an instance whereblack display changes into white display. As illustrated in FIG. 10,brightness is very low at the frame immediately after change from blackdisplay to white display, as compared with subsequent frames. Thisresult indicates that the response time (time required to reach 90% ormore of the brightness to be originally reached) is longer than the timeof one frame. When the response time is longer than the time of oneframe, an unwanted line-like pattern called “tailing” is seen uponscreen scroll (upon video display), which is a cause of loss of displayperformance. Conventional organic EL display devices having acompensation circuit, thus, had room for improvement in that thehigh-speed response characteristics of the organic EL element failed tobe brought out.

In the light of the above, it is an object of the present invention toprovide an active matrix substrate driven by an analog gray scalemethod, and an organic EL display device, in which decrease in responsespeed of a current emissive element is suppressed.

Means for Solving the Problems

The inventor conducted various studies on active matrix substratesdriven by an analog gray scale method in which decrease in responsespeed of a current emissive element is suppressed, and came to focus onregions in which a gate electrode of a transistor (driving transistor)that supplies current to a current emissive element is covered with apixel electrode of the current emissive element. The path of currentthat is supplied to a current emissive element from a transistor thatsupplies current to the current emissive element is preferably as shortas possible. Therefore, the current emissive element and the transistorare often disposed close to each other. In terms of securing an emissionregion that is as wide as possible, the area ratio of the pixelelectrode is normally set to be high. For the above reasons, the pixelelectrode of the current emissive element and the gate electrode of thetransistor that supplies current to the current emissive element areoften disposed overlappingly (in superposition), and parasiticcapacitance is thus likely to occur. The number of members disposed inthe pixel is substantial, and the layout of the various members iscomplex, in particular, in pixels that are provided with a compensationcircuit for compensating variability of threshold voltage in thetransistor that supplies current to the current emissive element.Therefore, the region in which the gate electrode of the transistor thatsupplies current to the current emissive element is covered with thepixel electrode of the current emissive element and is likely to becomelarger. Also, pixel layout is complex in a case where a compensationcircuit is made up of a plurality of transistors, as in the organic ELdisplay device illustrated in FIGS. 8 and 9. As a result, the pixelelectrode of the current emissive element covers more readily the gateelectrode of the transistor that supplies current to the currentemissive element, and in some instances, the pixel electrode may coverthe entirety of the gate electrode. Focusing now on the wiring layer, ascanning line is formed in a first wiring layer, and hence wiring (forinstance, a gate electrode 102 of the transistor T4 that suppliescurrent to the current emissive element) for forming capacitance and forconnection between transistors is normally formed at a wiring layerdirectly below a pixel electrode that is an overlying wiring layer. Inthe organic EL display device illustrated in FIGS. 8 and 9, parasiticcapacitance (hereafter notated as Cad) is formed between the gateelectrode 102 of the transistor T4 and a pixel electrode 103 (anode) ofthe organic EL element OLED. In the light of the measurement results ofFIG. 10, the inventor speculated that this Cad might be a cause of thestep-like response.

To verify the above study results, simulation measurements of responsewaveforms were performed in the organic EL display device illustrated inFIGS. 8 and 9, with changes in Cad. FIGS. 11, 12 and 13 are graphsillustrating response waveforms of current as obtained by simulationmeasurements of response waveforms in cases where Cad was 0, 20 and 60fF, respectively.

As illustrated in FIGS. 11 to 13, no step-like response is found whenCad is 0 fF; however, a step-like response arises for Cad of 20 and 60fF. The regions surrounded by the broken line in FIG. 12 and FIG. 13denote sites at which step-like response occurs. It is found that thedifference between current in the 1st frame and current in the 2nd frameincreases as Cad rises from 20 fF to 60 fF.

The relationship between Cad and current supplied to the organic ELelement was evaluated on the basis of the results of the above responsewaveform simulation. FIG. 14 is a graph illustrating the relationshipbetween Cad and current supplied to the organic EL element. FIG. 14reflects also the results of simulations performed for Cad other than 0,20 and 60 fF. In FIG. 14, the “current ratio” in the ordinate axisdenotes current ratio between the 1st frame and the 3rd frame afterswitching from black display to white display or half-tone display, andis a value resulting from dividing the mean value of current in the 1stframe by the mean value of current in the 3rd frame.

The results illustrated in FIG. 14 indicate that the current ratio tendsto decrease as Cad becomes greater. That is, the difference betweencurrent in the 1st frame and current in the 3rd frame tends to increaseas Cad becomes greater.

Brightness in the organic EL element is proportional to the current thatis supplied by the transistor that supplies current to the currentemissive element. That is, the current ratio in FIG. 14 is equal to thebrightness ratio in the 1st frame and the 3rd frame. Therefore, thecurrent ratio in FIG. 14 must exceed 0.9 in order to prevent theoccurrence of a step-like response characteristic through shortening ofthe response time to be shorter than the time of one frame. The resultsillustrated in FIG. 14 suggest that the current ratio exceeds 0.9 in acase where Cad is smaller than about 20 fF upon switching from blackdisplay to white display, and in a case where Cad is smaller than about16 fF upon switching from black display to half-tone display. In theorganic EL display device illustrated in FIGS. 8 and 9, however, thecurrent ratio was 0.9 or less, as is clear in FIG. 10, and the responsetime was longer than the time of one frame.

An explanation follows next on a driving method of the pixel illustratedin FIGS. 8 and 9, and on the reasons for the occurrence of a step-likeresponse on account of Cad. FIG. 15 is a timing chart of the 1st framein the pixel illustrated in FIGS. 8 and 9. In FIG. 15, displacement inthe vertical direction denotes voltage change in respective wirings,while displacement from left to right denotes passage of time. In FIG.15 the times in the various wirings have been arrayed vertically tofacilitate a comparison between the voltages at the respective wiringsat a same point in time. In FIG. 15, Vgs denotes the gate voltage of thetransistor T4.

Three steps, namely an initialization period a, a program period b andan emission period c, are sequentially provided in one frame, in thisorder. The various steps are explained next.

Firstly, in the initialization period a, the scanning line scan[n−1] isswitched on, and charge (data signal) stored in the capacitors C1, C2 isdischarged via the initialization voltage line Vini[n]. The gate voltageof the transistor T4 is initialized as a result.

In the program period b, next, the scanning line scan[n] is switched on,and data on each gray scale inputted from the signal line data iswritten in the transistor T4, to compensate thereby the thresholdvoltage of the transistor T4. At this time, the gate voltage of thetransistor T4 takes on a value that is lower, by the threshold voltage(Vth) of the transistor T4, than the voltage (V data) inputted from thesignal line data. Charge corresponding to the gate voltage of thetransistor T4 is stored in the capacitors C1, C2.

In the emission period c, the emission control line em[n] is switchedon, and current according to the gate voltage of the transistor T4, i.e.according to Vdata-Vth, is supplied to the organic EL element OLED,whereupon the organic EL element OLED emits light as a result.

An explanation follows next on the relationship between the gate voltageof the transistor T4 and the current that is supplied to the organic ELelement OLED from the transistor T4. FIG. 16 is a schematic diagramillustrating a TFT characteristic of the transistor T4 (transistor thatsupplies current to the current emissive element). In FIGS. 16, V8 (V)and V255 (V) denote, respectively, the gate voltage (Vgs) of thetransistor T4 in cases where a gray scale value is 8 and 255,respectively.

In the program period b, the threshold voltage of the transistor T4 iscompensated, and Vdata-Vth is set as the gate voltage of the transistorT4. In the emission period c, current flows according to the gatevoltage of the transistor T4. The gate voltage (Vgs) of the transistorT4 during emission obeys Vgs_1<Vgs_2 when a relationship V data_1<Vdata_2 holds. That is, the gate voltage (Vgs) of the transistor T4increases when the voltage (Vdata) inputted from the signal line dataincreases, and, as a result, a current value (Ids) that flows betweenthe source and drain of the transistor T4 becomes smaller. In the TFTcharacteristic illustrated in FIG. 16, Vgs_1 corresponds to V255 (V) andVgs_2 corresponds to V8 (V).

The reason for the occurrence of a step-like response on account of Cadis explained next. Focusing on the point in time at which the emissioncontrol line em[n] is switched on in the emission period c of FIG. 15,the gate voltage (Vgs) of the transistor T4 rises by a width denoted byα. This is deemed to arise from the capacitive component of the organicEL element OLED itself. At a non-display period (period at which theemission control line em[n] is off), charge cannot be drawn from thepixel electrode of the organic EL element OLED. When the emissioncontrol line em[n] is switched on, therefore, the Vgs of the transistorT4 rises up in the voltage direction of the previous frame, via Cad, toyield a voltage that is different from the original voltage.

From a next frame onwards, the voltage in the pixel electrode of theorganic EL element OLED results from adding the rise (or fall) fractionto the original voltage. Therefore, the voltage comes closer to theoriginal gate voltage, with less influence from a previous frame ascompared with the initial frame upon gray scale switching. The initialframe and the next frame exhibit thus a step-like responsecharacteristic upon gray scale switching.

It is found thus that Cad must be lowered in order to eliminate thestep-like response characteristic. As a result of further study, theinventor found that Cad is reduced, and the occurrence of step-likeresponse characteristic suppressed, by arranging the gate electrode of atransistor that supplies current to a current emissive element in such amanner that the gate electrode stands farther from a pixel electrode ofthe current emissive element. The inventor found that the above problemscould be admirably solved thereby, and arrived thus at the presentinvention.

Specifically, the present invention is an active matrix substrate drivenby an analog gray scale method, including a pixel that has a currentemissive element and a transistor that supplies current to the currentemissive element, wherein the pixel further has a compensation circuitfor compensating variability of threshold voltage in the transistor, thecurrent emissive element has a pixel electrode electrically connected tothe transistor, and a gate electrode of a transistor that makes up thecompensation circuit forms a region covered with the pixel electrode, apart or the entirety of the gate electrode that is positioned within theregion being provided in a wiring layer that is lower than a wiringlayer directly below the pixel electrode.

The configuration of the active matrix substrate of the presentinvention is not especially limited by other components as long as itessentially includes such components.

Preferable embodiments of the active matrix substrate of the presentinvention are mentioned in more detail below.

One preferred mode of the active matrix substrate of the presentinvention may be a mode wherein a part or the entirety of the gateelectrode is provided in a first wiring layer being a wiring layerclosest to the substrate. As a result, Cad can be sufficiently reduced,and the influence of a previous frame can be eliminated. Occurrence of astep-like response characteristic can be prevented as a result.

One preferred mode of the active matrix substrate of the presentinvention may be a mode wherein, in the active matrix substrate, ashield electrode is provided in a wiring layer between the pixelelectrode and a wiring layer in which the gate electrode is provided,and the shield electrode forms a region covering the gate electrode.This mode can be regarded as a mode in which the shield electrode isformed in such a way so as to cover the gate electrode. In such a mode,the shield electrode is formed between the gate electrode and the pixelelectrode. Hence, an organic EL display device can be provided that isexcellent in display performance and that is not influenced by theelectrodes at both ends that form the Cad.

The shield electrode can be connected to a power source line or thelike.

Pixel layout is complex in a case where a compensation circuit is madeup of a plurality of transistors, as in the organic EL display deviceillustrated in FIGS. 8 and 9. Therefore, the gate electrode 102 islikely to cover scanning lines and so forth that are formed at the firstwiring layer. In this case, therefore, the surface area of the portionof the gate electrode 102 that is formed in the second wiring layer(wiring layer directly below the pixel electrode 103) tends to increase,as does Cad. In the present invention, Cad can be reduced, and hence theproblem of the abovementioned configuration can be effectively solved.

One preferred mode of the active matrix substrate of the presentinvention may be a mode wherein the shield electrode is provided in thesecond wiring layer. The effect of the present invention can be broughtout yet more fully as a result.

One preferred mode of the active matrix substrate of the presentinvention may be a mode wherein the gate electrode forms a regioncovering a power source line. This mode allows reducing Cad andsuppressing the occurrence of a step-like response characteristic, andmakes it possible for capacitance (charge corresponding to the gatevoltage of the transistor T4) to be formed between wiring layers withoutproblems.

In a preferred mode, for instance, the gate electrode is formed at thefirst wiring layer and the second wiring layer, via a contact hole, apower source layer is formed at the first wiring layer, and a region isformed at which there overlap a gate electrode formed in the secondwiring layer and a power source line formed at the first wiring layer.

The present invention is also an organic EL display device provided withthe active matrix substrate of the present invention, wherein thecurrent emissive element in the pixel is an organic EL element, and thepixel electrode of the current emissive element in the pixel is an anodeor cathode of the organic EL element. In the active matrix substrate ofthe present invention, Cad is reduced, and occurrence of a step-likeresponse characteristic is suppressed. An organic EL display devicehaving excellent display performance can be realized as a result.

The aforementioned modes may be employed in appropriate combination aslong as the combination is not beyond the spirit of the presentinvention.

Effect of the Invention

By virtue of the active matrix substrate and organic EL display deviceof the present invention there can be provided an active matrixsubstrate driven by an analog gray scale method, and an organic ELdisplay device, in which decrease in response speed of a currentemissive element is suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a pixel of an organic ELdisplay device in an embodiment;

FIG. 2 is a plan-view schematic diagram illustrating a pixel in anorganic EL display device of Embodiment 1;

FIG. 3 is a cross-sectional schematic diagram along line Y-Y′ in FIG. 2;

FIG. 4 is a plan-view schematic diagram illustrating a pixel in anorganic EL display device of Embodiment 2;

FIG. 5 is a plan-view schematic diagram illustrating a pixel in anorganic EL display device of Embodiment 3;

FIG. 6 is a cross-sectional schematic diagram along line Z-Z′ in FIG. 5;

FIG. 7 is a plan-view schematic diagram illustrating a pixel in anorganic EL display device of Embodiment 4;

FIG. 8 is a plan-view schematic diagram illustrating a pixel of aconventional organic EL display device provided with a compensationcircuit;

FIG. 9 is a cross-sectional schematic diagram along line X-X′ in FIG. 8;

FIG. 10 is a graph illustrating measurement results of responsecharacteristic of a conventional organic EL display device provided witha compensation circuit;

FIG. 11 is a graph illustrating a current response waveform obtained bysimulation of a response waveform in a case where Cad is 0 fF;

FIG. 12 is a graph illustrating a current response waveform obtained bysimulation of a response waveform in a case where Cad is 20 fF;

FIG. 13 is a graph illustrating a current response waveform obtained bysimulation of a response waveform in a case where Cad is 60 fF;

FIG. 14 is a graph illustrating the relationship between Cad and currentsupplied to an organic EL element;

FIG. 15 is a timing chart of a 1st frame in a pixel of a conventionalorganic EL display device; and

FIG. 16 is a schematic diagram illustrating a TFT characteristic of atransistor T4 (transistor that supplies current to a current emissiveelement).

MODES FOR CARRYING OUT THE INVENTION

In the present description, “pixel electrode” denotes an electrodeelectrically connected to a drain electrode of a transistor thatsupplies current to a current emissive element, from among theelectrodes comprised in the current emissive element. In the case of anorganic EL element, the pixel electrode may be an anode or a cathode.

In the present description, the “current emissive element” is notparticularly limited and need only be an element that emits light byitself on account of supplied current. Examples of current emissiveelements in which the present invention is particularly effectiveinclude, for instance, planar current emissive elements such as organicEL elements, inorganic EL elements and the like.

In the present description, a “wiring layer directly below the pixelelectrode” denotes a first wiring layer, counting from the pixelelectrode, from among the wiring layers that are disposed furthertowards the substrate side than the pixel electrode. Ordinarily,interlayer dielectrics are disposed between the pixel electrode and thewiring layers. Therefore, a “wiring layer directly below the pixelelectrode” denotes also a “wiring layer adjacent to the pixel electrodeacross an interlayer dielectric”.

In the present description, a configuration wherein layer A and layer Bare at a same level indicates at least one instance from among aconfiguration wherein an underlying layer in contact with layer A and anunderlying layer in contact with layer B is a shared layer, or aconfiguration wherein an overlying layer in contact with layer A and anoverlying layer in contact with layer B are a shared layer, butpreferably denotes an instance where both such configurations aresatisfied. The wiring layer denotes a layer of wiring that is used as aconductor inside and outside the pixel. The first wiring layer denotes awiring layer closest to the substrate, i.e. a wiring layer that standsfirst, counting from the substrate, from among the wiring layers thatare disposed further on the pixel electrode side than the substrate.Similarly, the second wiring layer is the wiring layer that standssecond counting from the substrate. The wiring layer is ordinarilylow-resistance wiring (metal wiring). For instance a wiring layer thatforms a scanning line is the first wiring layer, and a wiring layer thatforms a data line is the second wiring layer. A semiconductor layer isordinarily present in the form of a semiconductor portion such as achannel or the like and a pattern-like portion having lowered resistancethrough ion implantation via a mask or the like. A semiconductor layerhaving thus a semiconductor as a material and that makes distinct use ofsemiconductor characteristics or conductor characteristics, depending onthe application, does not constitute a wiring layer in the presentdescription.

In plan-view schematic diagrams that illustrate a pixel of an organic ELdisplay device, the first wiring layer is depicted by a broken line andthe pixel electrode and the second wiring layer are depicted by solidlines. FIG. 8 and FIG. 9 illustrate a semiconductor layer 101 of a TFT,but the semiconductor layer 101 is depicted with a broken line.

The present invention will be mentioned in more detail referring to thedrawings in the following embodiments, but is not limited to theseembodiments.

(Embodiment 1)

FIG. 1 is a circuit diagram illustrating a pixel of an organic ELdisplay device according to Embodiment 1.

The arrangement relationship between the various members in the pixelillustrated in FIG. 1 is explained with reference to FIGS. 2 and 3. FIG.2 is a plan-view schematic diagram illustrating a pixel in an organic ELdisplay device provided with a compensation circuit of Embodiment 1.FIG. 3 is a cross-sectional schematic diagram along line Y-Y′ in FIG. 2.

Scanning lines scan[n−1], scan[n], scan[n+1], an emission control lineem[n] and an initialization voltage line Vini are formed in one samelevel (first wiring layer), and extend in the horizontal direction inFIG. 2. A signal line data, formed in a second wiring layer, extends inthe vertical direction in FIG. 2.

The first wiring layer is disposed at a level closer to a substrate 100than the second wiring layer. FIG. 2 and FIG. 3 illustrate aconfiguration wherein a gate electrode 102 is formed only at the firstwiring layer.

One pixel electrode 103 that functions as an anode of the organic ELelement OLED, is disposed at each region delimited by the scanning linescan[n−1], the scanning line scan[n+1], a power source line ELVDD andthe signal line data. This region, which functions as one display unit,will also be referred to, in the present description, as a pixel. Asemiconductor layer of the transistors T1 to T6 and the gate electrode102 of the transistor T4 are disposed in the pixel. The region denotedby A is a pixel opening that functions as the display region of theorganic EL display device.

As illustrated in FIG. 3, an interlayer dielectric 110, an interlayerdielectric 111 and an interlayer dielectric 112 are stacked, in thisorder, from the side of the substrate 100. The first wiring layer (gateelectrode 102 of the transistor T4) is disposed between the interlayerdielectric 110 and the interlayer dielectric 111. The second wiringlayer (power source line ELVDD and signal line data) is disposed betweenthe interlayer dielectric 111 and the interlayer dielectric 112. Thepixel electrode 103 is disposed on the interlayer dielectric 112. An endportion of the pixel electrode 103 is covered by an edge cover 113.Through covering of the periphery of the end portion of the pixelelectrode 103, the edge cover 113 allows preventing shorts between thepixel electrode 103 and the cathode (power source line ELVSS) that isdisposed opposite the pixel electrode 103 across an organic EL layer.The portion at which the edge cover 113 is not formed corresponds to theopening A.

In the organic EL display device of Embodiment 1, the gate electrode 102of the transistor T4 that supplies current to the current emissiveelement forms a region covered with the pixel electrode 103, and isprovided in a wiring layer that is lower than the wiring layer (secondwiring layer) directly below the pixel electrode 103. Forming the gateelectrode 102 this way allows reducing the capacitive component betweenthe pixel electrode 103 and the gate electrode 102, and allows loweringCad to 15 fF or less. Occurrence of a step-like response can besuppressed as a result, and there can be realized an organic EL displaydevice having excellent display performance.

As illustrated in FIG. 3, the signal line data and the power source lineELVDD are formed at the wiring layer directly below the pixel electrode103 (second wiring layer). The gate electrode 102 of the transistor T4is provided in a wiring layer that is lower than the second wiring layer(i.e. at the first wiring layer being a wiring layer closest to thesubstrate).

(Embodiment 2)

The circuit diagram illustrating a pixel of an organic EL display deviceof Embodiment 2 is identical to that of Embodiment 1.

FIG. 4 is a plan-view schematic diagram illustrating a pixel in anorganic EL display device of Embodiment 2. In the organic EL displaydevice of Embodiment 2, the gate electrode 102 of the transistor T4 isformed in a first wiring layer and a second wiring layer via a contacthole 105.

As illustrated in FIG. 4, the gate electrode 102 forms a region coveredwith the pixel electrode 103, and is provided in a wiring layer (firstwiring layer) that is lower than the wiring layer directly below thepixel electrode 103; as a result, this allows lowering Cad to 15 fF orless, and suppressing the occurrence of a step-like responsecharacteristic.

Also, capacitance (capacitor C1 formed through overlap of the gateelectrode 102 and the power source line ELVDD) that is formed betweenthe wiring layers can be formed without problems by virtue of thepresent configuration wherein part of the gate electrode 102 (portionformed at the second wiring layer) forms a region covering the powersource line ELVDD that is formed at the first wiring layer.

An organic EL display device having excellent display performance can berealized as a result.

(Embodiment 3)

The circuit diagram illustrating a pixel of an organic EL display deviceof Embodiment 3 is identical to that of Embodiment 1.

FIG. 5 is a plan-view schematic diagram illustrating a pixel in anorganic EL display device of Embodiment 3. FIG. 6 is a cross-sectionalschematic diagram along line Z-Z′ in FIG. 5.

In the organic EL display device of Embodiment 3, the gate electrode 102of the transistor T4 that supplies current to the current emissiveelement is provided in a first wiring layer that is lower than a wiringlayer (second wiring layer) directly below the pixel electrode 103.

As illustrated in FIG. 5 and FIG. 6, the gate electrode 102 forms aregion covered with the pixel electrode 103, and is provided in a wiringlayer that is lower than the wiring layer directly below the pixelelectrode 103; as a result, this allows lowering Cad to substantially 0fF, and suppressing the occurrence of a step-like responsecharacteristic.

As illustrated in FIG. 6, a shield electrode 104 is provided in a wiringlayer (second wiring layer) between the pixel electrode 103 and a wiringlayer (first wiring layer) that stands closest to the substrate and inwhich the gate electrode 102 is formed, such that the shield electrode104 forms a region covering the gate electrode 102 that is provided inthe first wiring layer.

Forming thus the shield electrode 104 at the second wiring layer abovethe gate electrode 102 allows preventing, yet more thoroughly,susceptibility to the influence of the electrodes at both ends where Cadis formed.

An organic EL display device having excellent display performance can berealized as a result.

The shield electrode 104 that is connected to the signal line data andthe power source line ELVDD is formed at a wiring layer (second wiringlayer) directly below the pixel electrode 103.

An end portion of the pixel electrode 103 is covered by the edge cover113. The portion at which the edge cover 113 is not formed correspondsto the opening A. Interlayer dielectrics 112, 111, 110 are formed,respectively, between the pixel electrode 103 and the second wiringlayer, between the second wiring layer and the first wiring layer (gateelectrode 102), and between the first wiring layer and the substrate100.

(Embodiment 4)

The circuit diagram illustrating a pixel of an organic EL display deviceof Embodiment 4 is identical to that of Embodiment 1.

FIG. 7 is a plan-view schematic diagram illustrating a pixel in anorganic EL display device of Embodiment 4. In the organic EL displaydevice of Embodiment 4, the gate electrode of the transistor thatsupplies current to the current emissive element is formed in a firstwiring layer and a second wiring layer via a contact hole 105.

As illustrated in FIG. 7, the gate electrode forms a region covered withthe pixel electrode 103, and is provided in a wiring layer (first wiringlayer) that is lower than the wiring layer directly below the pixelelectrode 103; as a result, this allows lowering Cad to substantially 0fF, and suppressing the occurrence of a step-like responsecharacteristic.

Also, capacitance (capacitor C1 formed through overlap of the gateelectrode and the power source line ELVDD) that is formed between thewiring layers can be formed without problems by virtue of the presentconfiguration wherein part of the gate electrode (portion formed at thesecond wiring layer) forms a region covering the power source line ELVDDthat is formed at the first wiring layer.

In FIG. 7, the shield electrode 104 is provided in a wiring layer(second wiring layer) between the pixel electrode 103 and a wiring layer(first wiring layer) in which part of the gate electrode (i.e. portionformed at the first wiring layer) is provided, such that the shieldelectrode 104 forms a region covering part of the gate electrode (i.e.portion provided in the first wiring layer).

Forming thus the shield electrode 104 at the second wiring layer abovethe gate electrode allows preventing, yet more thoroughly,susceptibility to the influence of the electrodes at both ends where Cadis formed.

An organic EL display device having excellent display performance can berealized as a result.

In the explanation above, TFTs and so forth have been omitted in thefigures, for the sake of an easier comprehension of the characterizingfeatures of the embodiments. In all embodiments, however, thecompensation circuit comprises a plurality of TFTs. The gate electrodeof T4 forms capacitance with the second wiring layer, but, in addition,can form capacitance with a semiconductor layer. In Embodiments 1 and 3,for instance, the gate electrode 102 of T4 can form capacitance with asemiconductor layer.

The aforementioned modes of the embodiments may be employed inappropriate combination as long as the combination is not beyond thespirit of the present invention.

The present application claims priority to Patent Application No.2009-241319 filed in Japan on Oct. 20, 2009 under the Paris Conventionand provisions of national law in a designated State, the entirecontents of which are hereby incorporated by reference.

Explanation of Reference Numerals

T1, T2, T3, T4, T5, T6: transistor

C1, C2: capacitor

OLED: organic EL element

scan[n−1], scan[n], scan[n+1]: scanning line

Vini[n]: initialization voltage line

em[n]:emission control line

ELVDD, ELVSS: power source line

data: signal line

A: opening

100: substrate

101: semiconductor layer

102: gate electrode

103: pixel electrode (anode)

104: shield electrode

105: contact hole

110, 111, 112: interlayer dielectric

113: edge cover

The invention claimed is:
 1. An active matrix substrate driven by ananalog gray scale method, comprising: a substrate, an organic ELelement, a power source line, a transistor that supplies current to theorganic EL element from the power source line, and a compensationcircuit for compensating variability of the threshold voltage in thetransistor, wherein the organic EL element has a pixel electrodeelectrically connected to the transistor, a gate electrode of thetransistor is provided in a first wiring layer and a second wiringlayer, a portion of the gate electrode in the first wiring layer iscovered with the pixel electrode, a portion of the gate electrode in thesecond wiring layer covers the power source line, the portions of thegate electrode in the first wiring layer and the second wiring layer areconnected via a contact hole provided in an insulating layer, the firstwiring layer is a layer closer to the substrate than the second wiringlayer, and the second wiring layer is a layer closer to the substratethan the pixel electrode.
 2. The active matrix substrate according toclaim 1, wherein a shield electrode is provided in a wiring layerbetween the pixel electrode and the first wiring layer, and the portionof the gate electrode in the first wiring layer is covered with theshield electrode.
 3. The active matrix substrate according to claim 2,wherein the shield electrode is provided in the second wiring layer. 4.An organic EL display device, comprising the active matrix substrateaccording to claim 1, wherein the pixel electrode is an anode orcathode.